A motion picture filmstrip generally includes a series of pictures and at least one soundtrack. Such filmstrips are typically used in conjunction with a motion picture projector to provide an optical effect wherein the pictures appear as a continuous picture having objects that are in motion. Typically, the motion pictures are projected on a screen. Many projectors further include means for reproducing the soundtrack in a synchronized manner with the picture projection. In this regard, reference is made to U.S. Pat. No. 4,009,949 entitled THREADING AND SYNCHRONIZATION FOR SOUND MOTION PICTURE PROJECTOR which issued to Nupnau on Mar. 1, 1977 and assigned to Bell & Howell Co. Therein is disclosed an automatic threading and image-sound synchronization system. In this system, the correct image frame to sound spacing of a filmstrip is established during threading of the projector by using a fixed loop accumulator and existing projector components.
Many techniques have been utilized to provide a soundtrack for a motion picture. One technique uses a magnetic medium similar to that used for magnetic recording tape. A soundtrack is then formed by applying the magnetic medium onto a motion picture filmstrip. However, this technique has disadvantages. A disadvantage is that the recording medium does not adhere well to the filmstrip. This results in a relatively short film life. In addition, the recording medium typically used is abrasive, which erodes and degrades the magnetic playback head, thus rendering it inoperable.
Another technique includes the formation of an analog optical sound recording on the filmstrip. One such recording is known as a variable area recording. This type of recording provides a soundtrack having optically translucent or opaque areas whose width varies according to the instantaneous amplitude of the sound. During movie projection, the soundtrack is illuminated from behind by a lamp and is viewed in front through a slit by a photocell. As the filmstrip advances, the photocell receives an amount of light that varies in accordance with the varying width of the varying translucent or opaque areas of the soundtrack. The photocell produces a corresponding voltage which is then utilized to reproduce the sound. In another type of optical recording, a soundtrack is configured such that it conforms to a curve that is empirically derived to provide the best compromise between signal to noise ratio, dynamic range and frequency response. This type of soundtrack is known in the art as the "Academy" soundtrack.
Digital optical recordings have also been used to provide a soundtrack. Generally, such recordings are formed by converting analog sound signals to digital sound signals. A filmstrip is then exposed to light which is modulated in accordance with the digital sound signals to form a digital soundtrack on the filmstrip. Digital soundtracks have many advantages. One advantage is increased resistance to audible degradation due to surface scratches or contaminants on the filmstrip that are caused by repeated use. In addition, digital soundtracks substantially reduce the amount of hiss and noise associated with analog soundtracks. As such, an increasing number of motion pictures include a digital optical soundtrack.
However, many movie theaters do not have projectors that are equipped to reproduce a digital optical soundtrack. Consequently, it is desirable to provide a motion picture having a standard analog optical soundtrack and a digital optical soundtrack. This would insure compatibility with theaters having projectors with either analog or digital sound reproduction systems. In this regard, reference is made to U.S. Pat. No. 4,306,781 entitled MULTI-CHANNEL SOUND AND DATA SYSTEM which issued on Dec. 22, 1981 to Mosely. This patent discloses a multi purpose sound/data system for motion picture film. As described therein, this system is capable of recording and reproducing digital and/or analog data such that there is no interference between different information channels.
U.S. Pat. No. 4,600,280 entitled DIGITAL AUDIO RECORDING/PLAYBACK SYSTEM FOR MOTION PICTURE FILM which issued to Clark on Jul. 15, 1986 describes a configuration for a filmstrip having digital and analog soundtracks. In this configuration, the filmstrip includes a three channel digital soundtrack extending along the length of filmstrip between the right sprocket holes and the picture frames. In addition, the film strip includes two conventional variable area soundtracks positioned between the left sprocket holes and the picture frames.
The filmstrip further includes a digital optical soundtrack which is formed in sublayers underneath the outside surfaces of the filmstrip. This soundtrack is formed on a portion of the filmstrip which includes an area between each of the righthand sprocket holes. The soundtrack further includes an area between each of the lefthand sprocket holes. In addition, the digital optical soundtrack may extend from the right edge to the righthand sprocket holes and/or the left edge to the lefthand sprocket holes. As such, the filmstrip is compatible with movie theaters having analog and digital sound reproduction systems. Another application having subject matter related thereto is PCT Application WO 92/14239, entitled STORAGE MEDIUM AND APPARATUS AND METHOD FOR RECOVERING INFORMATION FROM SUCH MEDIUM BY OVERSAMPLING, published Aug. 20, 1992.
Conventional projectors include a sound head for reading a soundtrack and reproducing sound in accordance with the soundtrack. In addition, such projectors include a drive sprocket and guide rollers positioned between a filmstrip supply reel and a take-up reel. The drive sprocket includes teeth which coact with drive sprocket holes in the filmstrip to advance the filmstrip past an aperture plate. Such projectors further include tension shoes for maintaining the filmstrip in contact with the drive sprocket. Additionally, projectors include a film trap having a pair of raised runners and pressure bands. The runners and pressure bands provide controlled movement of the filmstrip as the filmstrip is advanced past the aperture plate. Each edge of the filmstrip is placed in contact with each of the runners, pressure bands and tension shoes. Consequently, over a period of time the outside surfaces of both edges of the filmstrip become scratched or damaged as the filmstrip is advanced against the runners, pressure bands and tension shoes. These damaged surfaces may prevent a substantial portion of an optical soundtrack located in sublayers within the edges of the filmstrip from being read correctly by the sound head of the projector. This is a disadvantage since the soundtrack cannot be reproduced clearly or accurately by the sound head. By way of example, it has been determined that a filmstrip having a digital optical soundtrack located on each edge of the filmstrip is rendered unusable after the filmstrip is processed a maximum of approximately ten cycles through a conventional film trap. This amount is substantially less than the amount of cycles that the filmstrip may be processed before replacement if only the analog soundtrack of the filmstrip is used to reproduce sound. As such, use of the digital soundtrack to reproduce sound necessitates relatively early replacement of the motion picture and undesirably increases cost.
Consequently, it is an object of the present invention to provide a film trap which substantially reduces the amount of surface wear on each edge of a filmstrip. It is a further object to increase the number of cycles through which a motion picture filmstrip may be processed before replacement when using a digital optical soundtrack located within each edge of the filmstrip.